Masonry units, sometimes referred to as concrete masonry units (CMUs), include segmental retaining wall (SRW) units, pavers, and architectural blocks, all of which are made from a "masonry concrete," which is comprised of hydratable cement binder and relatively fine aggregate (e.g., particles of which less than 5% have a diameter of greater than 0.375 inch). Masonry concrete lacks the coarse aggregate (typically greater than 0.5 inch diameter) that is further used in conventional concrete, such as, for example, ready-mix concrete.
More significantly, masonry concrete differs from conventional concrete because it is said to have zero "slump," or, to phrase it more accurately, a practically immeasurable fluidity. The slump property is determined by compacting wet masonry concrete into an inverted cone and then removing the cone, and measuring the distance (if any) by which the cone-molded sample drops in height. See ASTM C143 (1988) ("Standard Test Method for Slump of Portland Cement Concrete"). Typically, the slump of masonry concrete is 0-0.75 inches, and thus masonry concrete is said to have essentially "zero slump."
The present inventors have discovered that the use of efflorescence control admixtures (ECAs), which are sometimes referred to as integral water repellants, such as calcium stearate dispersion, fatty acids, their salts or esters, oils, etc., may cause an appreciable loss of freeze-thaw durability in masonry units (e.g., made of zero slump masonry concrete). Efflorescence is the appearance of white, patchy stains on the face of masonry units during their manufacture or after their installation in the field. Once installed in the field, the masonry blocks are subjected to routine weathering conditions, one of them being alternate freezing and thawing (F/T) cycles. Recent testing of F/T durability of masonry units (SRW), using the ASTM C 1262 (1995) test method in the lab, revealed severe adverse effects of ECAs on the F/T durability of masonry units.
The present inventors therefore considered conventional air entraining agents (AEAs) which are known to increase F/T durability in conventional concrete. Conventional AEAs, such as gum rosin, VINSOL (tm) resin, and Tall Oil Fatty Acids, were evaluated for their ability to provide empty space within masonry concrete for receiving water that is displaced by ice formation, thus avoiding dangerous pressure-build up under freeze-thaw conditions. However, the inventors surprisingly discovered that conventional AEAs did not work in zero slump masonry units.
It should be mentioned here that AEAs operate to "entrain" air rather than to "entrap" it. AEAs stabilize air present in the mix (e.g., dissolved in mix water, folded-in and mechanically enveloped during mixing, present in intergranular spaces of cement, etc.). This is manifested by essentially spherical bubbles, typically between 0.001-1.25 mm. diameter, seen distributed relatively uniformly within the cement paste portion of the mix.
It is surmised by the present inventors that the nature of masonry units impedes the use of conventional AEAs because masonry concrete has smaller cement paste volumes than conventional concrete and because the masonry concrete mix is less fluid than a conventional concrete mix. This is also perhaps due to the fact that masonry concrete employs only relatively fine aggregate and has a zero slump characteristic. Combined with the fact that masonry concrete tends to have larger compaction void volume (i.e., interconnected voids which are not the same as entrained air voids), the very nature of masonry units thus defeats the efficacy of AEAs in entraining tiny air bubbles in the paste portion of masonry concrete mixes, and thus the attainment of F/T durability in masonry concrete is curtailed.
Accordingly, an improved masonry concrete admixture and masonry concrete having improved F/T durability are needed.